A Fresnel lens is a flat optical device that focuses light from a series of concentric grooves which define concentric facet surfaces and which are molded or cut into the surface of the device. Each groove is trapezoidal in cross-section, with the facet angles being varied in a controlled manner at different distances from the optical axis of the lens. Specifically, the Fresnel facets are formed on one surface of the optical zone of the lens, and no large thickness is required. This is because the Fresnel facets perform the desired focusing action by acting as a series of concentric prisms, directing the incoming light rays to a common point.
A Fresnel lens is used in the liquid crystal display (LCD) projection system of the present invention. The use of a single Fresnel lens as part of the imaging optical train is common practice for single-panel LCD projection systems. The Fresnel lens is generally employed as a field lens, located between the LCD spatial light modulator, which is usually referred to as the LCD valve, and the input to the projection lens assembly.
The purpose of a field lens is to "bend" the light rays passing through the LCD valve into a convergent cone, so that all the rays are captured by the input aperture of the projection lens. By having the field lens converge the rays between the LCD valve and the projection lens, the projection lens elements in the system can be smaller in diameter. Smaller diameter projection lens elements provide two principal advantages in the optical design.
The first advantage is a reduction in aberration of the projection lens elements. The second advantage is that a smaller lens is less costly to produce than a larger lens. This is because the cost of a simple lens varies roughly as the square of its diameter, so that halving the diameter of a lens reduces its surface area by a factor of four, thus reducing the work required to grind and polish its optical surfaces by 75%.
Since the purpose of the field lens is to redirect all the rays that pass through the LCD valve, the field lens must extend fully to the corners of the LCD valve. Therefore, the field lens must have a clear aperture which is at least slightly larger than the diagonal dimension of the LCD valve, assuming that the rays of light passing through the LCD valve are roughly collimated or only slightly divergent, as is typically the case. For a LCD valve with a diagonal measurement of 6.5 inches, the field lens must have a clear aperture approaching 7 inches, which is an extremely large lens.
If the field lens described above were not a Fresnel type, but a conventional glass lens, it would have to be relatively thick, heavy and expensive, and it would introduce substantial and undesirable field curvature. A Fresnel lens, however, is typically a thin (about 2 millimeters), injection-molded acrylic element, weighing 1 or 2 ounces, and costing a relatively small amount in production quantities. The major benefit in the use of a Fresnel lens as a field lens is that the thin, light, inexpensive optical element can be used to reduce substantially the size, weight, and cost of all the lens elements that follow the Fresnel lens in the optical train. Moreover, the Fresnel lens does not have the tendency to introduce undesirable field curvature.
However, complications arise in the use of Fresnel lenses in LCD projection systems. That is, as the focal length of the Fresnel lens becomes shorter and its f/# becomes smaller, the optic-mechanical features that make the Fresnel lens so useful are the same features that create potential pitfalls, by vignetting the light at the corners.
Accordingly, an object of the present invention is to provide a LCD projection system which utilizes Fresnel lenses, and in which the difficulties associated with the use of Fresnel lenses are obviated.